1. Field of the Invention
The present invention relates to an antenna apparatus having a ground plane and a linear antenna disposed on the ground plane.
2. Description of Related Art
As a system for radio-communicating between a base station and each of on-board terminal devices, a global positioning system and a system of an electronic toll collection are, for example, well known. In these systems, circularly polarized electric waves are used to reliably receive the electric waves in antennas of the terminal devices regardless of the coming direction of the electric waves. As antennas of the terminal devices receiving circularly polarized waves, patch antennas are often used. The patch antenna generally has a ground plane and a patch conductor disposed on the ground plane. The patch antenna is a narrow band and wide beam antenna. This antenna is, for example, disclosed in Published Japanese Patent First Publication No. 2001-267834.
The patch antenna is placed at a specific position of a vehicle from where electric waves radiated from the antenna can be transmitted without being reflected by any objects or disturbing the visual appearance of the vehicle, and where a visual field of the driver is not disturbed by the antenna. For example, the patch antenna is mounted on an upper side of an instrument panel, is incorporated into the panel or a rear view mirror, or is placed at a position near the mirror.
Further, because the antenna is placed in a narrow vehicle compartment, the antenna should be made in a small size, and the antenna should have specific radiation characteristics so as to sufficiently lower the intensity of electric waves radiated toward the back side of the antenna. These characteristics prevent the antenna from receiving unnecessary electric waves reflected by objects on the rear side of the vehicle compartment.
However, to sufficiently give these characteristics to the patch antenna, the patch antenna is required to have the ground plane set in an infinite size. Therefore, a small-sized patch antenna cannot sufficiently lower the intensity of electric waves radiated toward the back side of the antenna.